1. Technical Art
The present invention relates generally to an image display system using thin-film light emitting devices, and more particularly to an image display system of high image quality, which is suitable for an organic electro-luminescence (EL) display system.
2. Background Art
In recent years, display systems using organic EL devices are developed. When an organic EL device system comprising a multiplicity of organic EL devices is driven by an active matrix circuit, pixels of each EL are each connected with a set of FETs (field effect transistors) like thin-film transistors (TFTs) for controlling a current fed to each pixel. In other words, each pixel is connected with a set of a biasing TFT for feeding a driving current to an organic EL device and a switching TFT indicative of whether or not that biasing TFT is to be selected.
FIGS. 12 and 13 shows one example of the circuit diagram for a conventional active matrix type organic EL display system. This organic EL display system 310 are built up of X-direction signal lines X1, X2, . . . , Y-direction signal lines Y1, Y2, . . . , power source Vdd lines Vdd1, Vdd2, . . . , switching transistors (TFTs) Ty11, 12, Ty21, 22, . . . , current controlling transistors (TFTS) M11, M12, M21, M22, . . . , organic EL devices EL110, 120, EL210, 220, . . . , capacitors C11, 12, C21, 22, . . . , an X-direction peripheral driving circuit (shift register X-axis) 312, a Y-direction peripheral driving circuit (shift register Y-axis) 313, a screen 311, and so on.
A pixel is specified by X-direction signal lines X1, X2 and Y-direction signal lines Y1, Y2. At that pixel, switching transistors Ty11, 12, Ty21, 22 are put on, so that image data are held on signal holding capacitors C11, 12, C21, 22. This in turn puts on current controlling transistors M11, 12, M21, 22, so that biasing currents corresponding to the image data are passed through organic EL devices EL110, 120, EL210, 220 via power source lines Vdd1, Vdd2 for light emission.
For instance, as signals corresponding to the image data are produced at X-direction signal line X1 and Y-direction scanning signals are produced at Y-direction signal line Y1, switching TFT transistor Ty11 for the pixel specified thereby is put on, so that current controlling transistor M11 is brought into conduction by signals corresponding to the image data, whereupon a light emission current corresponding to the image data is passed through organic EL device EL110 for light emission control. In an active matrix type EL image display system comprising, per pixel, a thin-film type EL device, a current controlling transistor for controlling the light emission of the EL device, a signal holding capacitor connected to a gate electrode of the current controlling transistor, a switching transistor for writing data into the capacitor, etc., the light emission intensity of the EL device is thus determined by a current passing through the transistor that is a light emission current controlling non-linear device controlled by a voltage built in the signal holding capacitor (see A66-in 201pi Electroluminescent Display T. P. Brody, F. C. Luo, et. al, IEEE Trans Electron I devices, Vol. ED-22, No. 9, September 1975, P739-749).
Referring here to a full-color display system, light emission in various colors is achievable by making a selection from light-emitting organic EL materials. Alternatively, light emission in colors from blue to red is achievable by passing light emitted from an organic EL device through a color filter, provided that the organic EL device is made up of a material capable of emitting white light. As well known in the art, the full-color display system may be obtained by locating a display device emitting the three primary colors red, green and blue for each pixel.
In the case of a full-color display, its image signals are formed by a color image receiver. However, the signal amplitude ratio of each color obtained by the receiver is not necessarily in agreement with the ratio of a current value for driving each color of an EL device. Referring to the NTSC image signals as an example, white is represented by a ratio of red 0.3:green 0.59:blue 0.11. However, even though such signals are entered into a full-color EL display just as they are, it is then impossible to obtain white. This is because colors of light emitted from EL are delicately different from the NTSC red, green and blue and, at the same time, the current/luminance conversion efficiencies for colors from EL are not the same level.
An object of the present invention is to provide a color image display system which can achieve a proper color display and so form an image of high image quality, even when colors of light emitted from thin-film display devices are delicately different from NTSC or other image signals or the current/luminance conversion efficiencies for various colors are not the same level.
The aforesaid object is achieved by the inventions defined below.
(1) A color image display system comprising a thin-film display device driven by a current for each pixel 7 and designed to display colors corresponding to a plurality of color signals 3R, 3G and 3B, which further comprises:
a color signal conversion means 4 for converting the color signal ratio of said plurality of color signals 3R, 3G and 3B sent out of a color signal source 2 to the ratio of signals 5R, 5G and 5B suitable for the colors of said thin-film display devices.
(2) The color image display system according to (1) above, wherein said color signal conversion means is formed on the same substrate as that of said thin-film display device.
(3) The color image display system according to (1) or (2) above, which further comprises a light emission controlling device for feeding a driving current to at least said thin-film display device, and wherein said color signal conversion means is controlled in such a way that the input signal/output signal characteristics of said light emission controlling device correspond to each color to be displayed.
(4) The color image display system according to (3) above, wherein said light emission controlling device is a polysilicon TFT.
(5) The color image display system according to (3) or (4) above, wherein said input signal/output signal characteristics are a TFT transconductance.
(6) The color image display system according to any one of (1) to (5) above, wherein said thin-film display device is an organic EL device.